1. Field of the Invention
The invention is directed to optical amplifiers for wavelength division multiplexed (WDM/DWDM) optical networks, and in particular to a two stage modular fiber amplifier.
2. Background Art
The ITU has standardized wavelength-division multiplexed optical networks in which several optical channels are transmitted through a single optical fiber. Such networks can transport many signals, each transmitted over a separate carrier wavelength (channel), with each channel falling on one of the ITU grid frequencies. The ITU grid has frequencies spaced by 100 GHz, which corresponds to about 0.8 nm for wavelengths close to 1550 nm.
One of the advantages of WDM systems is that the optical component cost can be shared between all the transmitted channels. For example, current optical amplifiers, such as Erbium doped fiber amplifiers (EDFA) can simultaneously amplify a plurality of channels in the band from 1525 nm to 1610 nm.
Numerous problems are encountered in designing EDFAs for WDM networks. For example, the gain is not uniform across the WDM wavelength range of the EDFA. Therefore, the EDFAs exhibit a wavelength dependent gain, called gain tilt.
Gain tilt measures the change in the profile of the gain for each transmission channel at the actual value of the gain of the amplifier module with respect to the gain profile at the nominal value of the gain, i.e. at the value for which the amplifier is designed. For example, when the gain at 1550 nm is changed by 1 dB, the gain at 1530 nm changes by approximately 1.7 dB.
Gain tilt depends only on the physics of the dopant in the host fiber glass, and becomes a significant issue to consider in D/WDM networks. While no chemical solutions (dopants, fluoride, etc.) have yet been found for obtaining a flatt gain profile, electronic solutions are currently employed.
One known solution is to select the wavelengths for various channels amplified by the EDFA as a function of the gain variations of the different available pumps, so as to have similar gains for all channels. However, this solution becomes difficult when the number of channels is large.
Another solution is "gain clamping", which means maintaining the amplifier gain constant on all channels with an idler or lasing. However, this solution requires use of twice the number of laser pumps to provide the necessary extra photons.
Another solution is "loss padding", which implies tuning the loss of each span to match the nominal value for the amplifier or, in other words, to operate all amplifiers of the link at their nominal gain. Furthermore, this solution has the disadvantage of requiring variable optical attenuators (VOA) to be placed in each span, before or in the middle of the amplifier. This solution is not very robust in the presence of variations in losses and optical powers in the system over time and with temperature. Also, the system noise performance is limited to always be at the worst case. "Gain clamping" methods combined with "loss padding" slightly improve the robustness of the system, at the price of much more expensive pump lasers.
Another problem encountered in designing EDFAs for WDM networks is that, because the EDFA uses a single light source, the output power is shared among all channels, so that for N channels the output power/channel is roughly 1/N times the output for a single channel.
Still another problem in WDM networks is that stronger channels can saturate the EDFA gain, thereby limiting the gain of the weaker channels. This latter problem is of particular importance in ring, bus, and star networks, where channels propagate over widely varying distances. A solution to this problem is again to introduce VOAs in the stronger channels, which means additional equipment and power loss.
The prior art fails to provide cost effective solutions for amplification of bidirectional multi-channel optical signals.